USA 2019 Photo Workshop (XVIII) – Arches National Park
Arches National Park contains more than 2,000 natural sandstone arches are located in the park, including the well-known Delicate Arch and Landscape Arch, as well as a variety of unique geological formations. The park contains the highest density of natural arches in the world. The national park lies above an underground evaporite layer or salt bed, which is the main cause of the formation of the arches, spires, balanced rocks, sandstone fins, and eroded monoliths in the area. Deposited in Paradox Basin of Colorado Plateau over 300 mln years ago this salt bed is thousands of meters thick in places. Later, the salt bed was covered with debris eroded from the Uncompahgre Uplift to the northeast. During the Early Jurassic (about 210 Mya), desert conditions prevailed in the region and the vast Navajo Sandstone layer was deposited. An additional sequence of stream-laid and windblown sediments, the Entrada Sandstone (about 140 Mya), was deposited on top of the Navajo. Eventualy 1,500 m of younger sediments were deposited but these have been mostly eroded away. Remnants of the cover exist in the area including exposures of the Cretaceous Mancos Shale. The arches of the area are developed mostly within the Entrada formation. The weight of this cover caused the salt bed below it to liquefy and thrust up layers of rock into salt domes and “salt anticlines” – linear regions of uplift. In some places, they turned almost on edge. The result of one such 760 m displacement, the Moab Fault, is seen from the visitor center. Over time, water seeped into the surface cracks, joints, and folds of these layers. Ice formed in the fissures, expanding and putting pressure on surrounding rock, breaking off bits and pieces. Winds later cleaned out the loose particles. A series of free-standing fins remained. Wind and water attacked these fins until, in some, the cementing material gave way and chunks of rock tumbled out. Many damaged fins collapsed. Others, with the right degree of hardness and balance, survived despite their missing sections. These became the famous arches.